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Re: [Xen-devel] [PATCH v2 5/5] x86: Use alternative mechanism to define CLAC/STAC



On 29/05/2014 06:34, Feng Wu wrote:
This patch use alternative mechanism to define CLAC/STAC.

Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper@xxxxxxxxxx>

---
  xen/include/asm-x86/asm_defns.h | 24 +++++++++++++-----------
  1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
index 0b9f898..dd50b97 100644
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -9,6 +9,7 @@
  #include <asm/processor.h>
  #include <asm/percpu.h>
  #include <asm/cpufeature.h>
+#include <asm/alternative.h>
#ifndef __ASSEMBLY__
  void ret_from_intr(void);
@@ -165,26 +166,27 @@ void ret_from_intr(void);
  #define __ASM_STAC      .byte 0x0f,0x01,0xcb
#ifdef __ASSEMBLY__
-#define ASM_AC(op)                                       \
-        btl $X86_FEATURE_SMAP & 31,                      \
-        CPUINFO_FEATURE_OFFSET(X86_FEATURE_SMAP)+boot_cpu_data(%rip); \
-        jnc 881f;                                        \
-        __ASM_##op;                                      \
-881:
-
+#define ASM_AC(op)                                                     \
+        661: ASM_NOP3;                                                 \
+        .pushsection .altinstr_replacement, "ax";                      \
+        662: __ASM_##op;                                               \
+        .popsection;                                                   \
+        .pushsection .altinstructions, "a";                            \
+        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;       \
+        .popsection
  #define ASM_STAC ASM_AC(STAC)
  #define ASM_CLAC ASM_AC(CLAC)
  #else
  static inline void clac(void)
  {
-    if ( boot_cpu_has(X86_FEATURE_SMAP) )
-        asm volatile (STR(__ASM_CLAC) : : : "memory");
+    /* Note: a barrier is implicit in alternative() */
+    alternative(ASM_NOP3, STR(__ASM_CLAC), X86_FEATURE_SMAP);
  }
static inline void stac(void)
  {
-    if ( boot_cpu_has(X86_FEATURE_SMAP) )
-        asm volatile (STR(__ASM_STAC) : : : "memory");
+    /* Note: a barrier is implicit in alternative() */
+    alternative(ASM_NOP3, STR(__ASM_STAC), X86_FEATURE_SMAP);
  }
  #endif


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